The Coch-Y-Bonddhu is a terrestrial fly imitation.
Natural insects like ants & bees can be food for trout.
Imitation flies like Coch-Y-Bonddhu are available imitating the natural insects (Hymenoptera) which are very diverse in appearance. Most hymenopterans have two pairs of membranous wings. It is only usually the ant, grasshopper and bee that are regularly used as sucessful terrestrial imitations like Coch-Y-Bondhu for trout fishing.